The Customary Status of the Outer Space Treaty is not an All or Nothing Proposition, by Ian Perry

The following is a guest post by Ian Perry, an attorney who received a J.D. and an LL.M. in air and space law from the University of Mississippi School of Law and is pursuing a J.S.D. in space law with the University of Nebraska College of Law.  LM


When different nations enter into agreement on a topic they may do so by a treaty in which they explicitly spell out what they have agreed to. Customary international law on the other hand, involves an implicit agreement between states, is traditionally understood to gradually form over time, and is not always codified in a treaty. Customary international law is relevant to the Outer Space Treaty because of claims that the treaty has become customary international law, which could mean that it might apply to states that have never ratified it or have withdrawn from it.[i] As the following shows, existing state practice is insufficient to make the entire Outer Space Treaty customary international law.

What is meant by customary international law? Customary international law is a kind of international common law; it is a body of rules and principles said to arise informally from the general and consistent practice of nations. See RESTATEMENT (THIRD) OF FOREIGN RELATIONS LAW OF THE UNITED STATES § 102(2) (1987). Evidence of customary international law includes judgments and opinions of international tribunals, such as the International Court of Justice (whose judges are approved by the U.N. General Assembly and Security Council); judgments and opinions of other nations’ judicial tribunals; and scholarly writings. Id. § 103.[ii]

Customary international law may apply to a local region, to all countries, or to all countries that have not persistently objected. The amount of state practice required for a custom to form is not consistently defined in the relevant literature. Customary international law can exist in the absence of a treaty or alongside a treaty, and there are many scholars who find at least parts of the Outer Space Treaty customary international law.[iii] The fact that part of a treaty overlaps with customary international law does not necessarily mean the whole treaty is custom. Customary international law is generally described as arising out of state practice done out of a sense of legal obligation—a rule in one part of a treaty might meet this test in some situations without all parts and all applications of the treaty doing so. For example, in North Sea Continental Shelf, the International Court of Justice distinguished different types of precedent and indicated that state practice in the form of maritime delimitations between geographically opposite states provided insufficient precedent to establish a customary rule regarding lateral delimitations between adjacent states.[iv] Practice in earth orbit is not necessarily precedent for celestial bodies. One need not say that the whole Outer Space Treaty has the same status under international law. A particular provision of the Outer Space Treaty might embody customary international law with regard to activities in an area where there is more state practice (such as with regard to free movement in earth orbit), and not embody it with regard to all activities in a physically different area where there is less state practice. By way of illustration, ask the question: Do states know enough about operations on the surface of Mars to intend by compliance with the Outer Space Treaty now to give up a future right to withdraw from some of its provisions if they are found impractical there? The ICJ has found actions in coastal waters up to about 60 miles from shore to be less than sufficient to establish a norm for waters 120-200 miles from the coast.[v] Manned stations on other planets appear to be at least as distinct from earth-orbiting satellites as these different areas of the ocean are from each other. Accordingly, current state practice is generally insufficient to make even relatively long-standing implementations of the Outer Space Treaty establish customary law for all celestial bodies.

Relevant to seeing whether there is the “sense of legal obligation” sufficient for a customary rule (as well as to attempts to treat the Outer Space Treaty as in a different category than other treaties), it appears there was a lack of consensus among the initial negotiators of the treaty on the sort of consent needed to create customary international law. It seems likely they would have disagreed on whether the Outer Space Treaty or related U.N. resolutions would bind non-Party states.

For one example, see the statement of an Australian delegate to the Outer Space Treaty negotiations:

The starting point of the two draft treaties before the Sub-Committee was General Assembly resolution 1962 (XVIII), unanimously accepted as a statement of the principles by which States should be guided in the exploration and use of outer space. There were, however, different views as to the place of such a resolution of the General Assembly in the creation and development of international law. Probably no delegation would claim that by virtue of its adoption by the General Assembly a resolution ipso facto became part of international law. Certainly the delegation of Australia did not. The representative of the United States had referred to the well-known statement of his distinguished predecessor Mr. Stevenson to the effect that the United States regarded the Declaration of Legal Principles as a statement of accepted international law by which all had agreed to be bound. But different views had been expressed even in the Sub-Committee, as for instance by the delegation of France, with regard to the binding character of the principles contained in the Declaration. That being so, and given the pace of contemporary development, it was highly desirable that the terms of resolution 1962 (XVIII) should now be given binding legal form in an instrument that would automatically become, for the parties, one of the sources of international law. Such an undertaking would also afford an opportunity to make improvements in the text of the resolution itself, considered as a definitive statement of legal obligation.[vi]

This statement occurred in a context in which the United States took a stronger view of the legal weight of General Assembly Resolutions[vii] and was being accused of obstructing the development of binding treaty norms by Eastern Bloc delegations.[viii] Thus, many of the parties to the Outer Space Treaty appeared at the time it was being negotiated to take an approach to international law which focused on the need for explicit agreement by States.

Analysis of state practice and intent is a way of showing what states have implicitly agreed. The records of the Outer Space Treaty’s negotiation do not show consensus that its rules were customary at the time of the treaty. The Outer Space Treaty should be treated like other international agreements, and its rules should not be exempted from the analysis of state practice and intent to be bound that is otherwise required to determine the extent to which a treaty embodies customary norms. In other words, the space age started just a decade prior to the ratification of the Outer Space Treaty; there was almost no state practice outside of earth orbit at the time the Outer Space Treaty was ratified. Despite important strides in space technology over the last five decades, there is still very little state practice outside of Earth orbit today. There was not sufficient state practice to make the entire Outer Space Treaty codify customary international law when it was ratified, and there still is not sufficient state practice to make all the rules in the Outer Space Treaty customary international law. It would be incorrect to claim that every country in the world is bound by every rule in the Outer Space Treaty in every space activity.

The Outer Space Treaty allows a wider scope to commercial action than is sometimes supposed,[ix] but if, as space exploration matures, we find ourselves in a situation in which part of the Treaty appears unworkable, an appeal to customary international law should not be used to shut down a cost/benefit analysis of the Treaty.


[i] It may seem counterintuitive or contradictory to suggest that a state could be bound by a treaty from which it has withdrawn when that treaty has a withdrawal clause, but what potentially binds the withdrawing state is not the treaty per se, but a rule of law which overlaps with the treaty. A rough analogy might be found in a situation where a contract spells out duties between the parties and contains a clause allowing parties to end the contract—suppose such a contract contained provisions which were required by other laws (anti-fraud law, rules regarding detrimental reliance etc.)—a party might withdraw and end some of its contract obligations while still being bound to those portions of the contract which were required by other law. In the same way, a treaty regarding, say, the law of war might provide for parties to withdraw without supposing that everything mentioned in the treaty was legally permissible for withdrawing states.

[iii] The customary status of the Outer Space Treaty is often claimed, but there are currently a relatively limited number of places where the customary status of the Outer Space Treaty is discussed in detail in the published academic literature on space law. Bin Cheng’s Studies in International Space Law and Lyall and Larson’s Space Law: A Treatise contain some of the more detailed discussions that are published at present. I appreciate the opportunity to publish a sketch of some of the issues here, I have drawn on my as-yet-unpublished LL.M. thesis in writing this, as well as on a law journal article on international jurisprudence relevant to this subject which will hopefully be published within a few months.

[iv] North Sea Continental Shelf Cases (Federal Republic of Germany v. Denmark; Federal Republic of Germany v. Netherlands), I.C.J. Reports, 1969, 3, 45

[v] Maritime Dispute (Peru v. Chile), Judgment, I.C.J. Reports, 2014, 3; 47. Cf. Id. 44-45, 50-52, 56-58, 65, 66.

[vi] Manual on Space Law, Volume III, Travaux Préparatoires and Related Documents, Nandasiri Jasentuliyana & Roy S. K. Lee ed. (1981 Oceana Publications Dobbs Ferry, New York), [hereinafter Manual on Space Law, Volume III] 49-50

[vii] As Bin Cheng noted in his Studies in International Space Law, this was a bit different from the United States’ later position. See Manual on Space Law, Volume III, 37 for a relevant statement by Ambassador Goldberg (i.e., referring to a U.N. resolution predating the Outer Space Treaty: “The United States had taken the position that those principles constituted international law as it was accepted by the Members of the United Nations, and it was a source of great satisfaction that in the two and a half years since the adoption of the Declaration there had not been any disputes about the nine principles it contained.”).

[viii] Manual on Space Law, Volume III, 40, 41 for the Russian delegate. A little after this he stated that states did not have the right to regard their achievements in space as giving them a right to claim things or engage in military activities, but it appears that he did not regard this assessment of the “rights” of states as being embodied in then-existing rules of international law. Id. 42. For the Romanian delegate, see Id. 54

[ix] See e.g. Ian Perry, “Law of Space Resources and Operations on Celestial Bodies: Implications for Legislation in the United States,” Astropolitics 15:1 (March 2017)